A turbomachine is known, e.g. a high pressure compressor of an airplane turbojet, that includes at least one variable-pitch stator vane stage that is arranged circumferentially within a casing. Conventionally, the compressor is constituted by a rotor having a plurality of rotary blade stages that are offset axially, with a plurality of variable-pitch stator vane stages being installed between them. Each stator vane stage is thus constituted by a plurality of vanes arranged radially and circumferentially inside the casing that forms a kind of ring, the vanes being stationary in position but variable in orientation. The orientation of all of the vanes must be identical for the entire ring of variable-pitch stator vanes; it is modified as a function of operating conditions.
For this purpose, each vane has a control pivot projecting radially outside said casing, and each pivot is connected by a lever to a common control ring disposed coaxially around said casing and mounted to turn thereon. Generally, the ring is mounted to slide on the outside surface of the casing and it is actuated to turn about its own axis, which coincides with the axis of the turbomachine. Depending on the position of slider pads between the ring and the casing, and depending on the locations of anchor points for the control means, friction can represent up to 30% of the drive force required to move the set of vanes. The behavior of the ring itself during such movements can rise to problems. The ring is sometimes observed to ovalize, and that is harmful to operation thereof.
Generally, the ring is made up of a plurality of segments, e.g. two half-rings, that are assembled together around the casing. Thus, FR-2 125 012 describes a control ring made up of a plurality of segments assembled together by bridge-forming elements so as to form a complete ring. The bridge-forming elements are bolted between spaced-apart ends of the ring segments. Two ring segments and a bridge-forming element are assembled together by radial bolting. The bridge-forming elements are relatively flexible and act somewhat like flexible hinges that accentuate deformation of the ring. This can lead to dispersion amongst the angles of attack of the vanes that may be as great as two degrees of more. When the compressor forms part of a turboprop, this lack of accuracy increases the risk of the engine pumping.